The present invention relates to an automatic transmission system for a motor vehicle, and more particularly to an automatic transmission system for a motor vehicle driven by an alcohol engine using a mixed fuel of alcohol and gasoline, the transmission ratio of which is controlled in dependency on the concentration of the alcohol in the fuel.
In a conventional system for controlling an automatic transmission, a shift pattern of the transmission is formed in accordance with load on the engine or output torque and the load of the engine.
Japanese Patent Application Laid-open 63-115957 discloses a control system, the shift pattern of which is changed by a changeover switch provided on the engine for advancing or retarding the ignition timing in accordance with octane number of the fuel. The changeover switch is connected to shift pattern changing means. When fuel having a different octane number is used, the changeover switch is operated to advance or retard the ignition timing to select a shift pattern corresponding to the output power of the engine, thereby increasing the engine power and improving fuel consumption of the engine.
In the alcohol engine which is controlled to maximum efficiency, the calorific value of the alcohol is slightly larger than that of the gasoline in a theoretical air fuel ratio and the flaming speed of the alcohol is higher than the flaming speed of the gasoline. Consequently, as the concentration of alcohol in the fuel increases, the ignition timing of the engine approaches the top dead center so that thermal efficiency is increased, thereby increasing the engine power.
However, since the engine performance changes in accordance with the changing alcohol concentration, the driver feels uncomfortable.